It is known that battery-operated devices that are equipped with rechargeable batteries are, connected to a charging unit for recharging the battery. The charging units are often equipped in such a way that they can be used for fast charging of the battery. In fast battery charging, however, considerable heating of the battery occurs toward the end of the charging process, to indicate that the battery is charged. A temperature sensor is connected to the battery by a thermal coupling to protect the battery against damage, such as overcharging. The temperature sensor detects the battery temperature so that the charging unit is turned off once a limit temperature is reached.
In practice, this means that charging the battery requires at least a third line, which leads to a corresponding control input to the charging unit. During battery operation of the electric unit, this control line is not needed.
From German Patent Application 25 20 599 (A1), assigned to General Electric Co. a circuit arrangement for charging storage batteries is known in which the temperature of both the storage battery and its surroundings is measured The charging current is determined as a function of the temperature difference.
From British Patent 2,088,158, corresponding to U.S. Pat. No. 4,507,368 a battery unit is known in which a bimetallic switch is thermally coupled to the battery, and when a temperature limit value is reached, it can interrupt the current circuit to the battery, both in charging and discharging operation.